Inkjet printing apparatuses can be used for single-color or multicolor printing of a printing substrate, for example a single sheet or a belt-shaped recording material made of the most varied materials (for example paper). The design of such inkjet printing apparatuses is known; see for example EP 0 788 882 B1. Inkjet printing apparatuses that operate according to the Drop on Demand (DoD) principle have a print head or multiple print heads with nozzles comprising ink channels, the activators of which nozzles (controlled by a printer controller) excite ink droplets in the direction of the printing substrate, which droplets are directed towards the printing substrate in order to apply print dots there for a print image. The activators can thermally (bubble jet) or piezoelectrically generate ink droplets.
Given an inkjet printing apparatus operating according to the DoD principle, the ink that is used is adapted in terms of its physical/chemical composition to the print head; for example, the ink is adapted in terms of its viscosity. Given low print utilization, not all nozzles of the print head are activated in the printing process; and thus many nozzles are subject to downtimes (print pauses), with the consequence that the ink in the ink channel of these nozzles is not moved. Due to the effect of the evaporation out of the nozzle opening, the danger exists that the viscosity of the ink then varies. This has the consequence that the ink in the ink channel can no longer move optimally and escape from the nozzle. In extreme cases the ink in the ink channel dries completely and clogs the ink channel, such that a printing with this nozzle is no longer possible.
The drying of the ink in the nozzles can be prevented in that a printing from all nozzles takes place within a predetermined cycle. This cycle can be set corresponding to the print utilization. Individual points in the unprinted regions of the printing substrate can thereby be applied, or print dot lines can be printed between print pages. These methods can lead to disruptions in the print image.
Furthermore, the ink volume in the ink channel can be mixed via oscillation in order to achieve a uniform viscosity in the ink channel. This method fails given longer downtimes of the nozzles since only a limited ink volume is provided in the channel.
An inkjet printing apparatus in which ink fog occurring during printing is suctioned away between print head and printing substrate is described in JP 60184851 A. The suction device is arranged so that a suction flow arises between printing substrate and print head, which suction flow is directed perpendicular to the path or flight directions of the ink droplets and carries along the ink fog. The ink fog is suctioned into a suction chamber in which is arranged a filter that filters the ink from the air flow.
From US 2008/0143781 A1 it is known to intermittently activate a cleaning operation in which the nozzles of the print head eject ink droplets. For this the print head is panned from the printing position into a cleaning position, in which cleaning position the emitted ink droplets cannot arrive at the printing substrate but rather arrive in a capture container. The ink fog occurring in the cleaning operation is drawn off with the aid of a suction device. The print head is subsequently panned back into the printing position again.
A printing device with ink fog suction is known from EP 1 923 216 A2. In particular in the cleaning operation—in what is known as a free spray mode—the nozzles of the print head emit ink droplets that are suctioned away with a suction device. The ink fog arising in the cleaning operation is thereby likewise drawn off as well so that no contamination of the printing apparatus can arise. The suction device can also generate such a suction flow at the print head that no contamination of the printing apparatus can arise in the print operation. The suction device can also generate such a suction flow at the print head that, in the print operation, the ink droplets used for printing are barely deflected by the suction flow; in contrast to this, the ink droplets of smaller diameter that form the ink fog are deflected into a suction intake.